1. Technical Field
The invention relates to a linear drive with hydraulic amplification including a hydraulic cylinder, a piston connected with a first screw drive which forms a mechanical return, and a control valve for the pressure medium arranged along the longitudinal axis of the first screw drive. The control valve has a piston which is movable through a setting member. The setting member is connected at one end through a second screw drive with the first screw drive. A setting drive acts on the setting member. The linear drive can be used for the driving of a fuel injection pump or for the driving of the inlet and outlet valves of a combustion engine.
2. Background Art
From Swiss Patent 594,141 is known such a linear drive with hydraulic amplification in which the device shown is called a linear amplifier. This drive includes a hydraulic cylinder having a piston rod which transmits forces directly to the machine parts to be moved. In a piston of the hydraulic cylinder is attached a screw drive of which a nut is connected with the piston and a spindle with a control piston of a control valve. In one of the forms of execution shown, the spindle of the screw drive is designed in two parts by which a reduction of the rotating mass is provided. A second screw drive forms, at the same time, a security against overload. The control movements are produced by an electrical drive step-switch motor which sets the spindle of the screw drive in rotation. The rotating spindle is screwed into and out of the nut, and thus moves the control piston of the control valve supported on it. In this way, the inflow and outflow of oil to and from the hydraulic cylinder is regulated and the hydraulic piston is set in motion. The production of the rotary movement on the electric motor requires only slight energy, but nevertheless exerts great forces on the hydraulic piston. To compensate the axial movements between the rotor of the electric step motor and the spindle of the screw drive, a coupling must be connected between these elements to permit axial displacements of the spindle. This coupling has the disadvantage that the mass which must be set in motion by the motor is considerably increased. So that the exactness of the transmission of the switching movement from the motor to the spindle remains assured, the coupling must be designed as rigid as possible against torsion, which is associated with considerable difficulties. Through the rapid and frequent switchings, the coupling is under very strong load, which leads to a rapid wear and to loss of accuracy of the transmission of movement. In rapid drive processes, such as in the driving of fuel injection pumps and valves in combustion engines, requiring switching times in the range of fractions of a second electric step-switch motors cannot, in many cases, maintain the switching times. Improvements are possible through expensive technical measures, but lead to very expensive drives which still have the disadvantage of a short life.
The use of a linear drive with hydraulic amplification for the hydraulic driving of a fuel injection pump on a combustion engine is known from German Disclosure 3,100,725. In the linear drive is used a one-part spindle through which the drive is extremely prone to disturbance. When the hydraulic piston stops as a result of overload or using up the whole movement distance, the electric drive motor is overloaded and the spindle or the motor may be damaged. If the hydraulic piston stands still, the control piston of the control valve can be brought into another switching position only by means of the electric drive motor or by rotation of the spindle. An automatic setting back into the zero position, for example, is not possible. This arrangement is especially unsuitable in the case of lifting movements of the hydraulic piston against a solid stop, since the control drive is subject to intolerable loads.